A various kinds of electric squibs have been developed as squibs for gas generators for inflating air bags equipped in cars.
Such a squib usually has metal pins for electrically connecting to the external, and a heating element at the other ends of the metal pins for igniting an explosive.
With hitherto used ignition devices, bridging wires have been used for igniting ignition powder. Nichrome wires have been used as bridging wires. If wire diameters are too fine, the bridging wires could not be attached. When using bridging wires of diameters enabling the wires to be attached, however, the wires of such diameters generally have large heat capacities so that sufficient electrical charges must be stored in capacitors having great capacities for the purpose of igniting ignition powder only by energy of communication as is the case with a BUS system.
As another ignition device, moreover, a method has been known for forming a thick film resistor directly on a printed circuit board using the technique for producing the printed circuit board.
For example, Patent Document 1 has disclosed a squib using a printed circuit board on which a thick film resistor is directly formed, and a varistor is mounted on another part on the printed circuit board for the purpose of electrostatic protection.
Further, Patent Document 2 has disclosed a squib obtained by production steps of mounting a resistive heating element on a printed circuit board likewise, connecting a capacitor and a varistor to the printed circuit board by soldering, and further connecting the product thus obtained to electrode pins.
Although, these ignition devices have been improved to reduce the energy required for ignition in comparison with the case using the bridging wires, the reduction in required energy has remained insufficient.    Patent Document 1: Official Gazette of Japanese Patent Application Laid Open No. 2003-205.823    Patent Document 2: Official Gazette of Japanese Patent Application Laid Open No. 2000-108,838
On the other hand, the semiconductor bridge (SCB) is a generic designation of bridges produced using the semiconductor technique such as a sputtering, vapor deposition, and the like. Ignition devices using semiconductor bridges are of a very fine structure having narrow widths of lines as compared with those using bridging wires or printed circuit boards. Further, the ignition devices using semiconductor bridges utilize thin film bridges whose film thicknesses are of the order of a few microns so that it is possible to make their heat capacity smaller so as to have quick responsibility and ignition ability with low energy. The bridge wires require time period of 800 to 1,000 microseconds for heating ignition powder to its ignition temperature with electric current of 1.2 A so that the energy required for ignition is of the order of 2.9 mJ. In contract therewith, the semiconductor bridges enable ignition powder to be ignited by energy less than 0.44 mJ using discharge from a capacitor having a small capacity. With the SCB, since semiconductor production equipment such as for sputtering and vapor deposition may be used, it is possible not only to make the board remarkably smaller but also to make the heat capacity of heating part smaller and to accurately control the heat capacity, thereby stably producing squibs which are higher in ignition responsibility and enable ignition with low energy.
When using such semiconductor bridges having smaller heat capacities, however, there is a risk of erroneous ignition of the squib due to noise such as external static electricity.
Therefore, known in the art is arranging a capacitor in parallel with the semiconductor bridges for absorbing discharged static electricity in order to prevent the erroneous ignition due to the static electricity as disclosed in Patent Document 3.
Moreover, a squib having an IC built therein has been disclosed in Patent Document 4 in order to make it possible to perform an ignition by means of communication called BUS system.
With this ignition device, a second small semiconductor plate including integrated circuits is located on a header, and further sequentially stuck thereon are a first small semiconductor plate including a capacitor having stored energy for igniting an ignition element and a third small semiconductor plate including the ignition element.
Moreover, in order to remove noise such as external static electricity, a filter capacitor is attached between electrode pins outside the header.    Patent Document 3: Specification of U.S. Pat. No. 4,944,224    Patent Document 4: Official Gazette of PCT Japanese Phase Publication No. 2000-513,799